The manufacture of planar lightwave component (PLC) devices, as taught in U.S. Pat. Nos. 6,306,563B1, 6,555,288B1 and 6,496,637B2, involves the spinning, curing and patterning of multiple thin layers of core and cladding polymers upon a rigid silicon wafer substrate to make planar polymer waveguides. Such core and cladding layers typically comprise highly fluorinated polymers characterized by low optical loss at 1550 nm. Typically these fluorinated polymers are further characterized by refractive index less than 1.4 at 1550 nm.
In common commercial practice, a large number of PLC devices are fabricated simultaneously on a single silicon wafer and the wafer so treated is then cut apart in a process known as dicing into individual PLC devices. The dicing process scars the input and/or output ends of polymeric waveguides. In many instances it is difficult to polish smooth the scarred polymeric waveguide end; as a result the imperfections so introduced may be a permanent, lossy feature of the PLC device.
According to the practice of the art, a bundle of silica optical fibers is mated and aligned to the end of a polymer waveguide on the edge of the PLC device. The components are permanently attached by use of a thin layer of optical adhesive at the interface. Such optical adhesives are typically based on epoxy curing chemistry and are designed to match the refractive index of silica (1.456 at 1550 nm). While epoxy-based optical adhesives are common, it is desirable in the art to replace them.
Optical adhesives in common use today were developed to minimize the interfacial loss between silica optical fibers and components and are thus characterized by a refractive index of ca. 1.456 at 1550 nm, the refractive index of silica. However, those adhesives are not well suited for use in coupling highly fluorinated plastic optical components either to each other, or to silica components because they do nothing to reduce the effects of interfacial roughness at the plastic optical interface. The large refractive index contrast between the highly fluorinated polymer waveguide of the PLC device and common adhesives cause the facets on the surface of the polymer waveguide to stand out in the optical path and scatter incident light resulting in optical loss and noise. A transparent adhesive is desired which is characterized by a refractive index that matches the refractive index of highly fluorinated polymer waveguides. Which is typically in the range of 1.35-1.39 at 1550 nm.
One approach to the problem is to employ adhesives based upon fluorinated acrylate chemistry similar to that of well-known polymeric waveguides. However, those adhesives known in the art are subject to unacceptably high shrinkage during curing. In the art, prior to curing, the optical components being mated are carefully aligned to ensure maximum optical power transmission across the interface. After the components are aligned, the adhesive is cured. Low shrinkage of the adhesive layer during curing is highly desirable because it is critical that the optical components being bonded maintain their position of alignment during the adhesive curing process. An adhesive composition that exhibits high shrinkage during curing will cause the components it is bonding together to shift in relation to one another, and thus results in misalignment of the optical connection. The present invention provides a significant improvement to the art by providing a low refractive index, low shrinkage fluoroacrylate adhesive with low absorbance at 1550 nm wavelength.
U.S. Pat. No. 5,045,397 teaches the preparation of low refractive index polymerizable fluids from mixtures of low molecular weight fluorinated acrylates and discloses their utility as adhesives to bond pieces of glass.